This invention relates to the use of a single sensor probe and monitor to detect and measure transcutaneous gases at the surface of the skin. More specifically, this invention relates to a sensor probe having dual sensing electrodes responsive to the effects of two different transcutaneous gases on an ion solution separated from the skin surface by a selectively permeable seal having respective regions covering the corresponding electrodes.
It is known in the art to measure oxygen and carbon dioxide in the blood non-invasively by measuring the partial pressures of oxygen (pO.sub.2) and carbon dioxide (pCO.sub.2) in the adjacent body tissue. The measurement is done by means of a transcutaneous gas sensor having a electrodes covered by a selectively permeable seal in the form of a membrane. In the case of oxygen, the sensor is a Clark electrode, named after its inventor, Leland Clark. In a Clark electrode, an electrolyte is placed above the membrane and bridges the two electrodes. The membrane face of the sensor is placed against the skin of a patient and a voltage is applied across the electrodes. Oxygen in the tissue diffuses through the skin, through the membrane and through the electrolyte to the electrodes where it is electrochemically reduced by the applied voltage across the electrodes to cause an electric current to flow between the electrodes. The current produced by the reduction reaction, which can be metered and recorded, is a measure of the oxygen in the tissue.
In the case of carbon dioxide (CO.sub.2), the sensor is a Stow-Severinghaus electrode, named after its inventors, R.W. Stow and John Severinghaus. A Stow-Severinghaus electrode (sometimes called a Severinghaus electrode) is a pH electrode, i.e., it measures the pH of a solution. When CO.sub.2 is dissolved in the electrolyte it affects the pH of the solution. A pH electrode connected to a pH meter can measure the pH. Since pH is proportional to pCO.sub.2, the pH electrode can also measure C0.sub.2.
In a Severinghaus electrode, as in the Clark electrode used to measure oxygen passing through the skin, an electrolyte is placed above the membrane and bridges the two electrodes. The membrane face of the sensor is placed against the skin. Unlike the oxygen sensing Clark electrode, in the Severinghaus electrode used to measure pCO.sub.2, no voltage is applied across the electrodes. Carbon dioxide in the blood diffuses through the skin, through the membrane and through the electrolyte. The effect of the CO.sub.2 dissolving in the electrolyte changes the pH of the electrolyte thereby inducing a voltage (much like a battery) which is measured as an indication of the pCO.sub.2 in the body tissue.
It is also known to measure both oxygen and carbon dioxide with a single sensor probe utilizing a single measuring electrode (cathode). This results in a compromise since no single measuring electrode is optimum for use in measuring both oxygen and carbon dioxide. Hence the use of two separate measuring electrodes, each optimized for its respective gas, e.g., oxygen and carbon dioxide, has been found preferable.
Moreover, it is known to facilitate removal and replacement of the membrane in an oxygen or carbon dioxide electrode sensor probe through the use of a detachable fixation ring as set forth in U.S. Pat. No. 4,280,505 to Dali. When a single active electrode is employed in a single sensor probe to measure oxygen or carbon dioxide, or both oxygen and carbon dioxide, no problem is presented with respect to the use of a fixation ring. When two active electrodes are employed in a single sensor probe to measure oxygen and carbon dioxide respectively and simultaneously a problem is presented with respect to the use of a fixation ring. Since carbon dioxide electrodes differ from oxygen electrodes, and the membrane material best suited as permeable to oxygen is different from the membrane material best suited as permeable to carbon dioxide, the membrane assembly cannot be randomly angularly positioned with respect to the electrodes as is permissible where a single gas measuring electrode is used.
The present invention solves the aforementioned problem in providing for a sensor for monitoring first and second transcutaneous gases with the use of a single probe having dual measuring electrodes engaging respective different selectively permeable seal materials, e.g., membrane materials which can be mounted by means of a fixation ring that allows the membrane materials to reproducibly engage their respective measuring electrodes thereby accomplishing lateral and angular alignment with respect to eccentrically mounted oxygen and carbon dioxide electrodes.